In a boiling-water reactor (BWR) nuclear power plant, oxygen, hydrogen peroxide and the like generated through water radiolysis occurring in a radiation field are present in a coolant of a reactor primary system (a reactor primary system coolant).
On the other hand, it is known that stress corrosion cracking is caused, in the presence of oxygen or hydrogen peroxide, in a stainless steel or a nickel base metal used as a construction material of a nuclear power plant under such a high-temperature environment as to be in contact with a reactor primary system coolant having a high temperature of approximately 280° C. As a countermeasure for suppressing the corrosion of such a metal member, a conventional nuclear power plant practices a hydrogen injection technology in which hydrogen is injected from a feed-water system for lowering the concentration of the oxygen or hydrogen peroxide in the reactor primary system coolant. The effect of lowering the concentration of the oxygen or hydrogen peroxide is exhibited as lowering of a corrosion potential of the metal member in contact with the reactor primary system coolant.
As another method for suppressing the corrosion of a metal member in contact with a reactor primary system coolant in a nuclear power plant, a photocatalyst anticorrosion technology for forming a film composed of a photocatalyst on the surface of a metal member is known as described in Patent Document 1 and Patent Document 2. In this technology, the photocatalyst depositing to the surface of the metal member is irradiated with light of an ultraviolet region for lowering a corrosion potential of the metal member, to which the photocatalyst deposits, by an action of an electron activated through a photo-excitation reaction of the photocatalyst.
Incidentally, this photo-excitation reaction efficiently proceeds when a noble metal is present in the vicinity of the photocatalyst. Therefore, if a photocatalyst or a high-functional photocatalyst obtained by depositing a noble metal to the surface of a photocatalyst is precedently deposited onto the surface of a metal member and a photo-excitation reaction is caused by irradiating such a photocatalyst with Cherenkov light, that is, light generated in a reactor core and including light of an ultraviolet region, the corrosion potential of the metal member can be lowered during an operation of the nuclear power plant.
As still another method for suppressing the corrosion of a metal member in contact with a reactor primary system coolant in a nuclear power plant, a technology to inject a noble metal into the reactor primary system coolant is known as described in Patent Document 3. In this noble metal injection technology, a noble metal is periodically injected from a feed-water system during an operation of a nuclear power plant, so as to allow the noble metal to be incorporated into an oxide film formed on the surface of a metal member, and thus, the corrosion potential of the metal member is lowered.
In the noble metal injection technology, however, it is necessary to also inject hydrogen so that a hydrogen/oxygen molar ratio in the reactor primary system coolant can be 2 or more. This is because the corrosion potential of a metal member is difficult to lower by the noble metal injection technology if the hydrogen/oxygen molar ratio in the reactor primary system coolant is lower than 2. Besides, if the noble metal injection technology is employed, the corrosion potential of the metal member is little lowered in an upper portion of a reactor, and hence, it is difficult to exhibit the anticorrosion effect in a metal member disposed in the upper portion of the reactor. In this manner, in the noble metal injection technology, an operation for controlling the hydrogen/oxygen molar ratio in the reactor primary system coolant is complicated as well as the anticorrosion effect for a metal member disposed in an upper portion of the reactor is small.
On the contrary, in the photocatalyst anticorrosion technology, even if the hydrogen/oxygen ratio in the reactor primary system coolant is lower than 2 or there is an excessive amount of hydrogen peroxide in the reactor primary system coolant, the anticorrosion effect for a metal member can be exhibited by lowering the corrosion potential of the metal member through irradiation with the light of the ultraviolet region.
Besides, even in a portion such as an upper portion of the reactor in which the anticorrosion effect is difficult to exhibit by employing the noble metal injection technology, the corrosion potential of the metal member can be lowered by the photocatalyst anticorrosion technology.
Accordingly, as a method for suppressing the corrosion of a metal member in contact with a reactor primary system coolant of a high temperature in a nuclear power plant, the photocatalyst anticorrosion technology is preferably employed, and in particular, the photocatalyst anticorrosion technology using a high-functional photocatalyst obtained by depositing a noble metal to the surface of a photocatalyst is more preferably employed.